I've been using a Manta Ray for about 6 years and I absolutely love it...
Recently, when I attempt my higher lifts (150 lb, I am female, and squat sumo-style), people in the gym have told me that I am leaning over too far to the front. This, despite my attempts to have picture perfect form, stick my ass all the way out, and keep looking at the ceiling. It's happened more than once, with several different people telling me the same thing.
I am inclined to believe them.
I'm sure it's because the bar is too high and my body is attempting to regulate the center of gravity.
That said, I still love my Manta Ray and I think it has a place helping newbies learn to squat, especially those people who don't have the greatest upper body development or cushioning, but whose can otherwise push some decent poundages with their legs.
I plan to start weaning off the thing soon, and learning how to hold the bar lower across my back and also deal without having that extra surface area to spread the load. I suspect it will be difficult, as my shoulder has to come back further in order to grip the bar down there, and my rotator cuffs aren't in the best shape.